The definition is close up photography in such a way that the image projected on our camera’s sensor is life size (or close to).
So does that mean, if your lens has the word "macro" on it, that it is capable of life size? Not always! The reason being is that the brainiacs in the various marketing departments have cleverly used this term to describe the lens’ abilite to focus on a subject so that when a print measuring 6 in x 4 in is made, the subject is close to life size. This means that on a 35 mm sensor camera (regular film camera) also called (full frame on digital), the magnification needed is only 1 to 4. The magnification ratio is also printed on the lens and focusing distance will vary (read the manual). So the closer you can focus on a subject, the larger it will project onto the sensor achieving "macro" sizes.
This means that a true macro lens will give you at least a 1 to 1 magnification ratio. Since 1 to 1 magnification is achieved through better glass, macro lenses are higher in price and generally have a longer barrel.
There are cheaper alternatives to achieving macros such as extension tubes, close up filters, telephoto extenders and reverse mounting. Which we’ll only touch on because the pro’s and con’s as well as pricing are of a personal preference nature. Although there are technical concerns you should be aware of.